Obsoleted unit, to weak for sub/special, usable as a captain only if you don't have a better QCK lead (and only very new players don't have Eneru, so...). See below for more details on why he is weak.
Back when he was released, and for a long while after, he was quite good as one of the first 2.5x captain and the best QCK lead. Today, unfortunately, he is pretty much obsoleted, replacable by a superior Gladius Donquixote Pirates. Compared to Gladius, Ace has better HP, but worse ATK (and ATK is a better stat then HP), and crucially, Gladius has a great special, while Ace's special is weak. And even those who don't have Gladius are much more likely to run F2P Eneru, Lightning Incarnate 200,000,000 Volt Amaru as a lead instead.
To add insult to injury, early units like Ace have unevolved units with different special, so you cannot level his special up with his unevolved copies.
Socketing guide: Really? Don't bother. If you don't have LegendAce, socket ColoAce instead.
Together with Shanks, he was one of the first two RR units on Global when it premiered.